The invention relates to a method for positioning webs of film of a packaging device, according to which goods are introduced in a first web of film moving in the advance direction in troughs in this first web of film and then a second web of film is withdrawn from a supply of web against the resistance of a brake and is placed on the first web of film and bound with the same with the advance of the second web of film downstream of the supply of web being detected by a recognition means and set by varying the brake force of the brake in dependence on the signal from the recognition means so that in each case a repeat as a defined area of the second web of film comes to lie on the troughs in the first web of film. In addition, the invention relates to a packaging device for the performance of this method.
A method of this kind or a packaging device of this nature is generally known. Here, the first web of film is provided with the troughs into which the goods to be packed are introduced. Then, the second web of film is placed on this first web of film provided with the goods and welded thereto, either using vacuum technology or with inclusion of air, for example in a sealing station. Subsequently, the individual troughs with the surrounding area of film are separated from each other and in this way individual items of goods packages are produced. This packaging method is frequently used for foodstuffs, but may also be used for other goods. For the inscription of the packaging, generally the second web of film is imprinted, with this frequently taking place completely so that, for example, the packed goods may be protected against UV light. The imprint is divided into repeats according to the packaging length so that one repeat comes to lie over each trough.
For purposes of appearance and to guarantee an airtight welding, the top, second web of film is preferably under tension so that no folds occur during the later packaging. For this, the second web of film is withdrawn from a supply, which may, for example, be formed from a roll, against the restraining force of an adjustable brake. The second web of film, which is under tension, is then placed on the first web of film by means of guides and then welded, whereby adjustment of the brake force may be used to set the position and extension of the flexible web of film. In practice, for example, the imprint should be performed so the desired image only appears after a certain pretension of the second web of film.
The position of the individual repeats is detected by a recognition means, whereby a regulator may be used to set the brake force so that the rate of advance of the second web of film may be matched to that of the first web of film.
Although this is adequate for the general control of packaging machines, it has been found that slight positional deviations are frequently possible so that the repeats do not come to lie one hundred percent exactly over the troughs. In particular with packaging which is to be printed right up to the edge, this has been found to be disadvantageous since, on the one hand, the goods packaged in this way give a poorer impression of quality and, on the other, the positional deviation may mean that necessary information is no longer on the actual packaging, but is cut off during the later cutting process and so the goods are no longer marketable. For example, information such as the use-by date of a foodstuff or the weight must under no circumstances fall out of the area of the later packaging.
Therefore, it is the object of the invention to create a method for the positioning of the web of film and a packaging device for the exact and simple positioning of the repeats of the second web of film over the troughs of the first web of film.
This object is achieved according to the invention in that a second recognition means determines the exact position of the repeat relative to the first web of film and, in the event of dimensional errors, adjusts the position of the first recognition means relative to the direction of conveyance of the second web of film.
By way of the embodiment of the packaging method according to the invention, it is now possible to define an exact position by means of the second recognition means. If it is hereby determined that the position deviates from the set position, a regulating intervention will be performed by means of changing the position the first recognition means.
By way of the embodiment according to the invention it may be achieved that, by means of the control of the actual condition by the second recognition means, the position of the first recognition means is changed relative to the direction of conveyance of the second web of film. The result of this is that, due to the control circuit comprising the first recognition means and the brake force, the control circuit is correspondingly influenced. For example, if the second recognition means determines that the repeat is slightly ahead of the required position, i.e. has come to lie slightly ahead of the troughs, the front part of any printed image applied would be cut off. In this case, the position of the first recognition means is oriented backwards, which means changed against the direction of conveyance of the second web of film whereby the first recognition means records the repeat correspondingly earlier and then actuates the brake correspondingly more powerfully in order, by means of withdrawing less of the second web of film, to correct the position of the second web of film over the first web of film as appropriate.
The suggested method is in particular performed with packaging devices which operate fully automatically. However, the method is suitable in the same way, to be used in other manufacturing processes, for example, which in particular involve positioning two films one on the other. Usually, the first web of film, which takes the goods to be packaged, is described as the bottom web and the second web of film, which seals the troughs, is usually described as the top web.
The suggestion according to the invention achieves a self-adjusting system. When setting the machine, the top web no longer needs to be arranged with high precision in relation to the bottom web since the two recognition means in conjunction with the stepped intervention in the control circuit in question and the brake and the film""s intrinsic elasticity achieve the relevant positioning of the top web on the bottom web. Therefore, the process runs fully automatically.
However, the suggestion according to the invention not only accelerates a format change, the amount of adjustment work required is reduced by the suggestion according to the invention, it also provides greater stability during the operation, since it is been found that the tensile stress of the web of film withdrawn from a supply of web is not always identical during the process. For example, after production, the film is wound on a reel whereby once again the web is slightly stretched or wound on under tension. This dimension is not always identical along the direction of conveyance which means that a certain offset of the repeat takes place, even if the repeat was aligned exactly on the trough at the start. This effect is also automatically compensated by the suggestion according to the invention. As a result, the invention achieves less manual or personal positioning work when changing a web of film and higher stability, i.e. constant packaging quality.
Due to its intrinsic elasticity, the web of film may be offset to a certain degree by actuating the brake or releasing the brake so that on a relevant signal from the first recognition means, the controller may perform a control intervention. If the second optical recognition means recognises a positional deviation of the repeat of the second web of film relative to the trough in the first web of film, the first recognition means is slightly offset in such a way that the brake is either (slightly) released or prematurely applied more powerfully. If the brake is released, the elasticity of the film will ensure that the repeat is pulled forward slightly.
On the other hand, it is possible for the second web of film to follow the advance of the first web of film, i.e. for the repeat to be arranged slightly downstream of the position of the trough. In this case, the brake may be released, with a constant withdrawal force the rate of advance of the second strip of film will increase and hence the after-running will be made up again.
Here, it is possible to achieve a correction or exact positioning of the web of film either by means of its own elasticity, which obviously can only be stretched as far as its tensile strength, or, however, in that the advance or brake for the web of film are controlled so thatxe2x80x94without additionally exploiting the elasticityxe2x80x94the relative position of the top web of film on the web of film with the troughs is changed. Once again, this makes it possible to change the relative position of the first recognition means in the direction of conveyance of the second web of film.
To perform the said method, the recognition means must be able to determine the position of the repeat, which advantageously takes place by means of so-called control marks which the recognition means can differentiate from the remaining area of the web of film. Control means of this type could, for example, be (optical) markings on the web of film, however it may also be possible for the position of the repeats to be identified using certain imprints. The recognition of the position, in particular the position of the bottom strip of film from edges or other significant parts of the films is also possible. As an alternative to the use of optical, i.e. visible control marks, however, it is also possible for invisible markings, for example ultraviolet reflecting control marks etc. to be provided. Advantageously, obviously, the embodiment of the recognition means is matched to the control marks. However, it is also possible instead of recognition means to use other sensors such as, for example, Hall sensors or sensors which work in a different way, which are sensitive to suitable means used as control marks on the web of film.
The optical recognition means may be simple photocells which respond to a specific signal strength. Here, the light required to switch on the photocell may fall on the cell as either reflected or transmitted light. With a transparent packaging, for example, a beam of light, which may comprise a weak laser beam, may shine through the packaging, i.e. through the first and second webs of film, so that when a recognition mark is passed, this beam is interrupted for a defined time. With this embodiment, it is possible to provide a recognition mark on both the first and second webs of film.
To make it possible to distinguish which control marks belong to which web of film, the control marks may have different extensions so signals of different lengths are generated by the photocell. The embodiment of the method according to the invention means it is now possible not only to detect and regulate the rate of advance of the second strip of film by the first recognition means, but also to determine the relative position of the first strip of film relative to the second strip of film by means of the second recognition means.
In a preferred embodiment of the invention, it is provided that the second recognition means itself comprises several sensors arranged at intervals (in the direction of conveyance). An arrangement of this kind is, for example, favourable if the rate of conveyance is not known exactly and the second sensor can measure the speed or time and hence also the offset. Usually, the offset of the first recognition means is otherwise determined from the known direction of conveyance; for this, the recognition means supplies only one time value for this offset.
Instead of the photocells, other optical recognition means, for example, a line camera or another type of camera, may be used. The increasingly faster image processing enables a camera at a defined position to detect the second strip of film from above and, if the imprinting on the second strip of film so permits, also to determine the position of the underlying first strip of film. If, on the other hand, it is not possible to shine light through the second strip of film, the underlying first strip of film may also be detected by the use of a second camera. A subsequent evaluation of the image then permits the exact determination of the position of the two strips.
The second strip of film is usually imprinted with information relating to the goods to be packaged and with advertising motifs. However, it is also possible for this second strip of film not to be imprinted and to comprise either a transparent or a non-see-through, in particular a coloured, plastic. In these cases, the control mark may take the form of additional aids such as, for example, lines or other selectively arranged optical defects. For example, the repeat may be characterised by a local change in the reflection properties. A change of this kind may be achieved, for example, by applying a matt finish to an otherwise glossy surface. It is also, for example, possible to provide a reflector layer activated by ultraviolet radiation as a control mark.
In a further preferred embodiment of the packaging device, the welding device, which after the alignment of the first web of film relative to the second web of film seals the trough by bonding the two webs of film at sealing station, is triggered by the second optical recognition means. This means that the welding process is only triggered if the second optical recognition means indicates to the controller that the alignment of the two webs of film relative to each other corresponds to the specified condition. This automatically prevents inaccuracies occurring during the packaging.
In order to enable the packaging device to be adjusted for a plurality of possible dimensions, preferably the position of the first and the second recognition means is adjustable in the advance direction. The further development according to the invention ensures that the device may be converted for other repeat or packaging trough lengths in a simple way. Advantageously, this adjustability is monitored electronically and controlled mechanically or electromechanically. The position in the direction perpendicular to the advance motion may also be adjusted, thus enabling different widths of the webs of film to be processed.
It is suggested according to the invention that the second recognition means is arranged either upstream or downstream of the sealing station. The advantage of an arrangement downstream of the sealing station is that here the top and bottom webs are already welded together and the actual condition, namely the condition of the finished packaging, may be reliably checked. Here, the disadvantage is the fact that the packaging is already closed and is possibly to be treated as scrap. Conversely, the use of the second recognition means upstream of the sealing station permits a certain possibility for correction so that the films in the sealing station are bonded correctly.
The advantages of bonding the second web of film, i.e. the top web, with the first web of film under tensile stress have already been referred to in the introduction. This avoids the formation of folds or waves.
In a preferred embodiment of the invention, it is provided that the degree of the positional correction of the first recognition means substantially corresponds to the dimensional error determined by the second recognition means. Here, an interval of approx. 70% to 130% of the known dimensional error may be specified, whereby this is in particular dependent upon the elasticity of the film and from the type and nature of the control circuit. Alternatively, it is provided that the degree of the positional correction of the first recognition means is limited by the dimensional error as the maximum value. Here, it is also possible for the correction to be performed in much smaller correction steps relative to the dimensional error in order gradually to approach the correct alignment.
Hereby, it is advantageous for the positioning method to be performed periodically with the period being longer than the processing time which passes while one repeat is conveyed from the first to the second detection means. If, in one control circuit, the control cycle is selected to be shorter when a first change has propagated itself in the system, then fluctuations or perturbations occur in the system and the desired result is not achieved. The control circuit must, therefore, have a certain xe2x80x9cinertiaxe2x80x9d which in this case is determined by the distance between the two recognition means on the one hand and the mean rate of conveyance. The further development according to the invention ensures that a change is not ordered too prematurely again by the controller before the first correction is checked. Under some circumstances, this length may extend to the distance between the second recognition means from the brake, as there are new properties in this area.
Further features and advantages of the invention may be found in the dependent claims and in the following description of a preferred embodiment with reference to the drawing showing a packaging device according to the invention.